The Mars rover Spirit has completed its first full set of scientific measurements with the instruments on its robotic arm, revealing mysterious hollow grains in the soil. The one-metre arm used its microscope to take a close-up image of an undisturbed patch of soil next to the NASA rover. It shows mostly sand-sized particles, but with a large number of apparently hollow spheres or tubes. The image resolution is about 30 microns per pixel – about the width of a human hair.